U.S. Pat. No. 5,525,843 discloses a seat occupant detection system that uses a seat heater element as an electrode of a capacitive sensor, wherein a pair of relay activated switches are used to either isolate or connect the heating element from or to the associated current supply and ground, wherein the heating element is isolated when sensing the occupant and is otherwise connected when heating the seat. One potential limitation with such an arrangement is that relay controlled switch elements may not be sufficiently reliable for safety applications, such as controlling the actuation of a safety restraint system.
A vehicle may contain safety restraint actuators that are activated responsive to a vehicle crash for purposes of mitigating occupant injury. Examples of such automatic safety restraint actuators include air bags, seat belt pretensioners, and side curtains. One objective of an automatic restraint system is to mitigate occupant injury, thereby not causing more injury with the automatic restraint system than would be caused by the crash had the automatic restraint system not been activated. Generally, it is desirable to only activate automatic safety restraint actuators when needed to mitigate injury because of the expense of replacing the associated components of the safety restraint system, and because of the potential for such activations to harm occupants.
One technique for mitigating injury to occupants by the air bag inflator is to control the activation of the inflator responsive to the presence and/or position of the occupant, thereby activating the inflator only when an occupant is positioned outside an associated at-risk zone of the inflator. NHTSA data suggests that severe injuries due to close proximity with the inflator can be reduced or eliminated if the air bag is disabled when the occupant is closer than approximately 4 to 10 inches from the inflator door. Such a system for disabling the air bag inflator requires an occupant sensor that is sufficiently sensitive and robust to make such a determination, while not causing the air bag inflator to be disabled when otherwise required for providing occupant restraint.
One technique for detecting the presence and/or position of an occupant is by sensing the influence of an occupant upon an electric field generated proximate to a seat for which the presence and/or position. An occupant has associated dielectric and conductive properties that can influence an electric field, and accordingly the occupant is an electric-field-influencing medium that can be detected with an electric field sensorxe2x80x94sometimes also known as a capacitive sensor.
As used herein, the term xe2x80x9celectric field sensorxe2x80x9d refers to a sensor that generates a signal responsive to the influence of a sensed object upon an electric field. Generally, an electric field sensor comprises at least one electrode to which is applied at least one applied signal; and at least one electrodexe2x80x94which could be the same electrode or electrodes to which the applied signal is appliedxe2x80x94at which a received signal (or response) is measured. The applied signal generates an electric field from the at least one electrode to a ground in the environment of the at least one electrode, or to another at least one electrode. The applied and received signals can be associated with the same electrode or electrodes, or with different electrodes. The particular electric field associated with a given electrode or set of electrodes is dependent upon the nature and geometry of the electrode or set of electrodes and upon the nature of the surroundings thereto, for example the dielectric properties of the surroundings. For a fixed electrode geometry, the received signal or signals of an electric field sensor are responsive to the applied signal or signals and to the nature of the environment influencing the resulting electric field, for example to the presence and location of an object with a permittivity or conductivity different to that of its surroundings.
One form of electric field sensor is a capacitive sensor, wherein the capacitance of one or more electrodes is measuredxe2x80x94from the relationship between received an applied signalsxe2x80x94for a given electrode configuration. The technical paper xe2x80x9cField mice: Extracting hand geometry from electric field measurementsxe2x80x9d by J. R. Smith, published in IBM Systems Journal, Vol. 35, Nos. 3and4, 1996, pp. 587-608, incorporated herein by reference, describes the concept of electric field sensing as used for making non-contact three-dimensional position measurements, and more particularly for sensing the position of a human hand for purposes of providing three dimensional positional inputs to a computer. What has commonly been referred to as capacitive sensing actually comprises the distinct mechanisms of what the author refers to as xe2x80x9cloading modexe2x80x9d, xe2x80x9cshunt modexe2x80x9d, and xe2x80x9ctransmit modexe2x80x9d which correspond to various possible electric current pathways. In the shunt mode, a voltage oscillating at low frequency is applied to a transmit electrode, and the displacement current induced at a receive electrode is measured with a current amplifier, whereby the displacement current may be modified by the body being sensed. In the xe2x80x9cloading modexe2x80x9d, the object to be sensed modifies the capacitance of a transmit electrode relative to ground. In the transmit mode, the transmit electrode is put in contact with the user""s body, which then becomes a transmitter relative to a receiver, either by direct electrical connection or via capacitive coupling.
A seat-based capacitive sensor can be affected by seat heater when co-located in the same area of the seat as the seat heater. A seat heater is typically a low resistance conductor (i.e. approximately 1xcexa9) in an under layer of the seat cover. In operation, a direct current of several amperes is sent through the heater element to generate heat. The heater element appears to be a ground to a capacitive sensor measuring the impedance to ground with an oscillatory or pulsed voltage.